Gas lasers have long been in use as sources of highly-collimated light. Until recently there was little alternative to such lasers when a beam of coherent collimated light was desired. Recently, however, semiconductor diode lasers operating in visible wavelengths have become available. Such lasers offer advantages over the traditional gas discharge lasers, particularly in that they are much smaller in size.
A significant problem with visible wavelength diode lasers when compared with gas lasers is the much lower degree of collimation that they exhibit. This problem is compounded by the fact that the divergence angle will be different in different dimensions, i.e. the beam is elliptical. This is further compounded by the fact that the diode lasers will exhibit astigmatism, that is the apparent origin of the divergence will be different in different dimensions. In order to use such lasers in typical situations, a system for either collimating or focusing the light is required.